Levels of Spinal Cord Injury

People with spinal cord injury are often told that they have an injury at a given spinal cord level, that they are “complete” or “incomplete”, that they have a bony fracture at one or more spinal vertebral levels, and that they are classified as A, B, C, D, or E according to the American Spinal Injury Association (ASIA) Classification. What is the meaning of the different spinal cord injury levels, the definition of complete and incomplete injury, and the different classification of spinal cord injury? In this article, I will try to explain the currently accepted definitions of spinal cord injury levels and classification.

Vertebral vs. Cord Segmental Levels
The spinal cord is situated within the spine. The spine consists of a series of vertebral segments. The spinal cord itself has “neurological” segmental levels that are defined by spinal roots that enter and exit the spinal column between vertebral segments. As shown figure 1, the spinal cord segmental levels do not necessarily correspond to the bony segments. The vertebral levels are indicated on the left side while the cord segmental levels are listed for the cervical (red), thoracic (blue), lumbar (green), and sacral (gray) cord.
The spine has 7 cervical (neck), 12 thoracic (chest), 5 lumbar (back), and 5 sacral (tail) vertebra. The spinal cord is shorter than the spinal canal, usually ending just below the L1 vertebral body. The C1 spinal roots exit the spinal column above the C1 vertebral body. There is no C8 vertebral body and so the C8 roots exit between C7 and T1. The T1 roots exit between T1 and T2 and the L5 roots exit between L1 and S1 vertebrae.
The first and second cervical vertebra hold and pivot the head. The C1 vertebrae, upon which the head is perched, is called Atlas after the Greek god who holds the earth. The back of the head is the occiput. The junction between the occiput and atlas is thus the atlanto-occiput junction. The C2 vertebra, upon which Atlas pivots, is called Axis, The junction between C1 and C2 vertebra is the atlanto-axis junction. The cervical cord innervates the diaphragm (C3), the deltoids (C4), biceps (C4-5), wrist extensors (C6), triceps (C7), wrist extensors (C8), and hand muscles (C8-T1).
The twelve thoracic vertebrae have associated ribs. The spinal roots form the intercostal (between the ribs) nerves that run on the bottom side of the ribs and connect to the intercostal muscles and associated dermatomes. About 5% of people have a vestigial 13th rib. The spinal cord ends just below L1. The conus is the tip of the spinal cord. Below the conus, the spinal roots of L2 to S5 form the cauda equina. Injuries to the lower thoracic spinal cords generally damage the lumbar enlargement. Injuries to the lumbosacral spine invariably reults in damage to the lumbosacral enlargement.

In summary, spinal vertebral and spinal cord segmental levels are not necessarily the same. In the upper spinal cord, the first two cervical cord segments roughly match the first two cervical vertebral levels. However, the C3 through C8 segments of the spinal cords are situated between C3 through C7 bony vertebral levels. Likewise, in the thoracic spinal cord, the first two thoracic cord segments roughly match first two thoracic vertebral levels. However, T3 through T12 cord segments are situated between T3 to T8. The lumbar cord segments are situated at the T9 through T11 levels while the sacral segments are situated from T12 to L1. The tip of the spinal cord or conus is situated at L2 vertebral level. Below L2, there is only spinal roots, called the cauda equina.